Chapter 11: Organic Chemistry - College of Science

[Pages:10]Chapter 11: Organic Chemistry. Saturated Hydrocarbons

11.1 Organic and Inorganic Compounds

Historically, organic chemicals have been associated with living systems. This association

originally derived from the belief that only living entities (i.e. plants and animals) could make

organic chemicals. Indeed, until 1828 all attempts to prepare any organic chemical from

exclusively inorganic reagents failed.

Before we go further, we need to define what an "organic" chemical is, as compared to an

"inorganic" chemical. Unfortunately, there is no ironclad dividing line between these classes of

chemicals. Nonetheless, we have a few guidelines that work for the vast majority of compounds.

(You'll see few, if any, exceptions.)

1) Organic compounds always contain only p-block elements (Groups III-VII), at least one

of which must be carbon.

2) Organic compounds almost always contain one or more C-H bonds.

3) Organic compounds are almost always molecules (as opposed to salts). Thus, all bonds

are typically covalent in organic compounds.

Methane (CH4) is the prototypical organic molecule. Stick drawings of methane and some other organic molecules follow.

H

O

O

C

H

H

H

methane

(natural gas)

C

H

H3C

O

acetic acid (vinegar)

C

H3C

CH3

acetone (nail polish remover)

O2N C

CH3

C

NO2

C

C

C

H

C

H

NO2

2,4,6-trinitrotoluene (TNT, explosive)

CH2

H

H3C

O

ethanol (gasohol, beer)

H

O

H

C

C

C

C

OH

C

C

CH3

H

C

C

H

O

2-acetylbenzoic acid (acetylsalicylic acid,aspirin)

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Although uncommon, there are organic compounds that don't contain a C-H bond. For

example, CCl4 is almost always classified as organic. There are two reasons for this. First, the

series CH4, CH3Cl, CH2Cl2, and CHCl3 are all organic and CCl4 is simply the final member of

the series, and second, in nearly all respects it behaves chemically like the other compounds in

this group. Likewise, although it is an ionic compound, [(CH3)4N][CH3CO2]

(tetramethylammonium acetate), would typically be considered an organic compound. (We will

get to how to name organic compounds later.)

It is interesting that the very first "organic" compound prepared from exclusively inorganic

reagents is now considered an inorganic compound. In 1828, Freidrich W?hlers decomposed

ammonium cyanate by boiling an aqueous solution of the chemical and obtained urea, a major

constituent of urine.

NH4NCO H2O

O H2NCNH2

This experiment caused others to begin examining whether organic chemicals could

generally be prepared from inorganic chemicals and it was quickly shown that there was nothing

special about living systems in the synthesis of organic compounds.

Some general properties of organic compounds include:

1) Like all molecular compounds, organic molecules typically have low melting points (in fact

many are liquids at room temperature). This is because London forces and dipole-dipole

interactions are usually the forces acting between molecules (p. 161).

2) They tend to have low molecular polarities.

3) Poor water solubility. Few organic molecules are readily soluble in water. (Although the 3

oxygen containing molecules on p. 1 are quite water soluble. We'll see why later.)

4) Poor electrical conductivity. Few pure organic substances conduct electricity well. (There

3 are no ions to help carry the charges.)

11.2 Some Structural Features of Organic Compounds

Communication is central to any field including chemistry. The structures you saw earlier

have the advantage of conveying a large amount of information about the spatial arrangement of

the atoms in a molecule. Each (except methane) contains some condensed structural

information. The problem with these structures is that they take up a lot of space, which makes

their use in normal text cumbersome. Furthermore, even when used to convey structural

information, they take a good deal of effort to write out. In this section we will discuss a number

of different ways to write out molecular formulas and see how they provide information to the

reader.

Let's begin with methane, CH4. You already know that hydrogen only forms 1 bond and

that carbon tends to form 4 bonds. Thus the most reasonable structure is one in which you have

4 hydrogens, each connected to the carbon by a single bond. Furthermore, from Chapter 4.8, p.

105, you know that molecules with 4 bonds around a central atom with no lone pairs on the

central atom are tetrahedral in shape. Because of this there is no good reason to draw a picture of

methane, as opposed to simply writing out CH4.

Ethanol is different however. If we simply write out C2H6O, it turns out there are two

different structures that can drawn from this molecular formula:

HH ..

H C C O.. H

H

H

.. H C O.. C H

HH ethanol

H

H

dimethyl ether

4 Thus a simple molecular formula isn't all that helpful in this situation. In fact, they are rarely useful in organic chemistry. We can use a slightly expanded molecular formula and still get a good deal of structural information, however. If we write ethanol as CH3CH2OH and dimethyl ether as CH3OCH3 we get the same information as in the pictures if we apply the rules for drawing Lewis structures to them and we do so using less space. Another shortcut we can use involves repeating groups. Consider the molecule octane:

HHHHHHHH HCCCCCCCCH

HHHHHHHH

We could write this out as CH3CH2CH2CH2CH2CH2CH2CH3, but this too is cumbersome and is prone to either the writer or reader miscounting the number of CH2 groups. These same types of problems exist when writing the formulae of cyclic molecules as text. We can shorten this long string to CH3(CH2)6CH3. Again, if we apply the rules of drawing Lewis structures, there is only one way to draw the structure of this molecule.

When drawing structural formulas, there is a different simplification organic chemists employ. We have already seen that organic compounds tend to have a lot of C-H bonds. Thus we simplify by assuming that carbon forms four bonds and that any bond to carbon that is not explicitly shown is a C-H bond. We simplify one step further by not bothering to actually draw in the letter C. In this case, octane becomes:

In this structure, each vertex is a carbon atom. The terminal carbons each have one C-C bond so they must have 3 C-H bonds as well. The middle carbons each have 2 C-C bonds, and

5 so have 2 C-H bonds. Using this notation we can simplify some of the structures shown on the first page to

O2N

NO2

NO2

2,4,6-trinitrotoluene

O

acetone

O H

O

acetic acid

H O

ethanol

Although you may find that it takes a little while to get used to drawing structures this way,

you will find it makes your life much simpler in the long run.

We now turn to some odds-and-ends of discussing structures. First the octane molecule

shown earlier is a straight chain structure. Don't take this as literal truth. In fact each carbon is

tetrahedral, and if each CH3 in octane were grasped and pulled apart, a structure much like the

drawing above (for octane, with the zigzag line) would result. When CxHy groups appear off the

"straight chain" a branched chain molecule results. Thus 2-methylheptane would look like:

CH3

or

CH3CHCH2CH2CH2CH2CH3

CH3

or

CH3CH(CH2)4CH3

or (CH3)2CH(CH2)4CH3

Double and triple bonds are usually written explicitly. For example, propene can be written out in any of the following ways:

HHH

HCCC

H

H

CH

H3C

CH2

CH3CH=CH2

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There exists free rotation about single bonds. In other words, if you had CH3Cl and grasped the chlorine atom, the CH3 group would spin like a propeller. What this means is that in a flask containing octane, all of the molecules aren't strung out like the picture, but rather they twist and coil and constantly reorient themselves. This will become important when we get to biochemistry.

11.3 Isomerism Molecules possessing the same molecular formula but exhibiting different structures are

called isomers. One of the reasons we use structural formulae is to show this explicitly. We have seen two examples of this so far. The first was shown explicitly, ethanol (CH3CH2OH) vs. dimethyl ether (CH3OCH3) (p. 3 of the notes). Can you figure out the other (answer next paragraph)?

There are actually several types of isomers but the only one we will be concerned with now are constitutional isomers. These are molecules that have the same numbers and types of atoms, but different atomic connectivities. We will see other types of isomers later. The other isomers are octane and 2-methylheptane (both are C8H18).

The properties of isomers may be very similar to one another (as is the case for octane & 2methylheptane) or they may be quite different (see Table 11.1, p. 335 of your book for ethanol vs. dimethyl ether).

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11.4 Functional Groups

The basic unit of organic chemistry is a molecule consisting of only carbon and hydrogen

with only C-C and C-H single bonds. The variety found in organic chemicals largely derives

from the replacement of hydrogen atoms with other groups, or the presence of C-C double and

triple bonds. Collectively, these collections of atoms are called functional groups. A table (11.2)

of important functional groups appears on p. 337 of your book. We will discuss most of these in

Chapters 12 ? 16 and all of them in the biochemistry chapters.

There are a few points worth mentioning here. Molecules possessing the same functional

groups frequently exhibit similar properties. For example, amines are molecules possessing an

-NH2 group. Just like ammonia (NH3) is water soluble, CH3NH2 (methyl amine) and

CH3CH2NH2 (ethyl amine) are water soluble. Both organic amines have unpleasant odors

(actually worse than NH3) and form basic solutions when dissolved in water just like ammonia. Another feature of functional group chemistry is that the functional groups affect properties

less as the molecules become larger. Let's look at the solubility of alcohols in water as the

organic groups get larger:

Alcohol

CH3OH CH3CH2OH CH3CH2CH2OH CH3CH2CH2CH2OH CH3CH2CH2CH2CH2OH CH3CH2CH2CH2CH2CH2OH

Solubility (per 100 g of H2O at 20 ?C) any amount any amount any amount 7.9 g 2.7 g 0.6 g

When a molecule contains more than one of the same functional group, the effect of the functional group on properties typically becomes more pronounced. Again let's look at a series of alcohols.

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Compound CH3CH3 CH3CH2OH HOCH2CH2OH

Boiling Point -88 ?C 78 ?C 197 ?C

As we will see later in the semester, when a molecule contains two or more different functional groups the changes in physical properties may roughly average or unusual effects may occur.

11.5 Alkanes and Cycloalkanes The simplest, most fundamental class of organic molecules is the alkanes. These are

molecules consisting only of carbon and hydrogen with only C-H and C-C single bonds. Those alkanes that form rings are called cycloalkanes. Methane (CH4) and ethane (CH3CH3) are alkanes. Cyclohexane (C6H12) is a cycloalkane. It can be represented by either of the two structures below. The second structure gives a more accurate picture of how the atoms are arranged in 3-dimensional space.

or

Alkanes are important as fuels (CH4 = natural gas, C3H8 = propane (LPG), C4H10 = butane (disposable cigarette lighters), C5H12 ? C10H22 = gasoline, higher order alkanes form kerosene and airplane fuel (as well as candle wax), and lubricating oils.

Alkanes and cycloalkanes are saturated hydrocarbons. By saturated we mean that each carbon atom is bound to the maximum possible number of hydrogen atoms. That is, there are no

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